Photobioreactor And Uses Therefor

ABSTRACT

The present invention provides novel photobioreactors, modules thereof, and methods for use in culturing and harvesting algae and cyanobacteria.

CROSS REFERENCE

This application is a continuation of U.S. patent application Ser. No.13/273,161, now U.S. Pat. No. 8,241,895, filed on Oct. 13, 2011, whichin turn is a continuation of U.S. patent application Ser. No.12/280,338, now U.S. Pat. No. 8,198,076, filed Aug. 17, 2009, which inturn is the National Stage under 35 U.S.C. §371 of InternationalApplication No. PCT/US2007/004351, filed Feb. 20, 2007, which claims thebenefit of priority of U.S. Provisional Patent Application Nos.60/775,174, filed Feb. 21, 2006 and 60/799,930, filed May 12, 2006, eachof which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

Microalgae and cyanobacteria (for short, algae) are micro-plants andrequire mostly simple mineral nutrients for growth and reproduction. Byutilizing photon energy, such as sunlight and artificial illumination,algae convert, through photosynthesis, water and carbon dioxide intohigh-value organic compounds (e.g., pigments, proteins, fatty acids,carbohydrates, and secondary metabolites). Algae exhibit a growthpotential an order of magnitude greater than higher plants because oftheir extraordinarily efficient light and nutrient utilization.

With over 40,000 identified species, algae represent a very diversegroup of organisms. They naturally produce many novel, as yet largelyuntapped classes of bioproducts. Globally, annual sales of algae-derivedproducts (pharmaceuticals, nutraceuticals, agrochemicals, human food,and animal feed) were estimated to be $2 billion in 2004. By takingadvantage of the latest breakthroughs in molecular biology, metabolicengineering and functional genome research, algae can serve as anexcellent gene-expression vehicle for production of recombinant proteinsand other biologically active compounds for human and animal health andnutrition.

Due to the ability to rapidly uptake nutrients (such as carbon dioxide,nitrogen, and phosphorous) from the surrounding environment and convertthem into organic compounds such as proteins stored in the cell, algaehave been proposed and tested in natural and engineered systems toremove and recycle waste nutrients from wastewater and carbondioxide-rich flue gases emitted from fossil fuel-fired power generators.The algal biomass produced as a by-product of the bioremediation processcan then be used as feedstock for production of biofuels (such asbiodiesel, ethanol, or methane), animal feed additives, and organicfertilizer.

Although application of algae for renewable biofuels of both liquid andgaseous forms and high-value products, and for environmentalbioremediation is scientifically and environmentally sound, economicviability of algal applications is determined by the efficiency andcost-effectiveness of industrial-scale culture vessels, or so-calledphotobioreactors (for short, reactors), in which algal grow andproliferate.

Industrial photobioreactors currently are commonly designed as openraceways, i.e. shallow ponds (water level ca. 15 to 30 cm high) eachcovering an area of 1000 to 5000 m² constructed as a loop in which theculture is circulated by a paddle-wheel (Richmond, 1986). Thisproduction mode has the advantage of being relatively simple inconstruction and maintenance, but it has many disadvantages which relateto the factors controlling productivity of algal grown outdoors(Richmond, 1992; Tredici et al. 1991). The overall low productivity ofthe open raceways is due mainly to the lack of temperature control andthe long light-path, as well as poor mixing. The open raceways in whichthe algal culture is open to the air is also responsible for culturecontamination with airborne microorganisms and dusts, often causingculture failure or crashes. The significant drawbacks of the openraceways have prompted the development of closed systems, i.e.photobioreactors made of transparent tubes or containers in which theculture is mixed by either a pump or air bubbling (Lee 1986; Chaumont1993; Richmond 1990; Tredici 2004).

A number of tubular photobioreactors have been proposed and developedsince the pioneering works of Tamiya et al. (1953) and Pirt et al.(1983). These solar receptor bioreactors are generally serpentine orhelical in form, made of glass or plastic with a gas exchange vesselwhere CO₂ and nutrients are added and O₂ removed connected to the twoends of the tubing, and with recirculation of the culture between thevessel and tubing performed by a pump (Gudin and Chaumont 1983) or anair-lift (Pirt et al. 1983; Chaumont et al. 1988; Richmond et al. 1993).Because of their improvement in light path, culture temperature, andmixing, tubular photobioreactors not only increase considerably algalbiomass productivity, but also enable more algal species of commercialinterest to grow and proliferate under more controllable cultureconditions.

On the other hand, the tubular-type photobioreactors suffer from theirown inherit problems. First of all, tubular photobioreactors have asignificant ‘dark zone or dark volume’ (usually consisting of 10-15% ofthe total culture volume) associated with a degas reservoir/tank wherethe exchange of excess amounts of dissolved oxygen with carbon dioxideoccur. Algal cells entering the dark zone cannot perform photosynthesis,but consume, through cellular respiration, cell mass which havepreviously been assimilated under light. As a result, a tubular reactorwill only sustain biomass yield of 85-90% of the theoretical maximum.Secondly, tubular photobioreactors have the potential to accumulate inthe culture suspension high concentrations of molecular oxygen evolvedfrom photosynthesis, which in turn inhibits photosynthesis and thusbiomass production potential. Thirdly, mechanic pumps that are commonlyemployed by tubular photobioreactors to facilitate culture mixing andcirculation within long tubes can cause serious cell damage. Forexample, some 15% of the damage to cells has been reported to beassociated with operation of tubular-type bioreactors (Shilva et al.1987). Gudin and Chaumont (1991) also observed that significant cellfragility occurred in a Haematococcus culture maintained in alarge-scale tubular photobioreactor. Due to severe hydrodynamic stresscreated by various mechanical pumps, only a limited number of algalspecies are able to survive in a tubular bioreactor. Also, the highcapital and maintenance costs associated with tubular photobioreactorshave limited their applications only for production of small quantity,high-value specialty products.

During the last ten years, however, attention has focused on flatplate-type photobioreactors. This type of reactor was first described bySamson and Leduy (1985) and by Ramos de Ortega and Roux (1986), andfurther refined by Tredici et al. (1991, 1997) and Hu et al. (1996,1998a,b). Flat plate-type designs offer greater advantages over thetubular-type systems: 1) no “dark zone” is associated with theflat-plate design and the reactors are illuminated in their entirety,thus boosting photosynthetic productivity; 2) aeration that facilitatesculture mixing and turbulence exerts little harm to algal cells becauseof the minimum hydrodynamic force created by air bubbling; 3) harmfullevels of oxygen are not built up in flat plate-type system because oftheir short reactor heights (i.e., 3 to 10 feet); 4) flat-plate reactorscan be set at various orientations and/or tilted angles aimed at maximalexposure to solar energy throughout the year to further enhancephotosynthetic biomass yield; and 5) compared to tubular reactors,flat-plate reactors require considerably less capital and maintenancecosts.

However, application of flat plate-type reactors has encountered a majorengineering obstacle, i.e., difficulty of scaling up the flat plat-typedesign to a commercial level. Therefore, flat plate reactors have onlybeen used as bench-top culture devices and as small outdoor cultureunits for study of algal growth physiology, and have never been appliedto industrial cultivation of algae.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides photobioreactors,comprising:

a container adapted for holding fluid, comprising

-   -   opposing first and second sidewalls, wherein at least one of the        first and second sidewalls is transparent;    -   opposing first and second endwalls;    -   a container bottom; and    -   a container cover,    -   wherein the first and second sidewalls comprise a plurality of        separate sections, and wherein the separate sections are in        fluid communication;

support struts for connecting the plurality of separate sections of thefirst and second sidewalls;

at least one inlet port in fluid communication with the container;

at least one outlet port in fluid communication with container;

an aeration system in fluid communication with the container; and

a temperature control system connected to the container so as to controltemperature of fluid within the container.

In one preferred embodiment, the photobioreactor further comprises oneor more baffles connected to the first and second sidewalls, so as toform a barrier and to partially separate the container into multiplecompartments.

In another embodiment, the invention provides photobioreactor modules,comprising two or more photobioreactors of the first aspect of theinvention, wherein the containers of each photobioreactor are in fluidcommunication. Individual photobioreactor containers can also be set inparallel and the fluid from individual photobioreactor containers can beharvested through outlet ports connected to a common harvesting/drainingmanifold system.

In a second aspect, the present invention provides a photobioreactorpanel unit, comprising:

a container adapted for holding fluid, comprising opposing first andsecond sidewalls and opposing first and second endwalls, wherein thecontainer define an interior, a top opening and a bottom opening,wherein at least one of the first and second sidewalls is transparent,and wherein the first and second sidewalls are substantially flat;

a top cap that fits over the top opening of the panel body; and

a base cap that fits under the bottom opening of the panel body;

one or both of the top cap and the bottom cap further comprise one ormore channels, to provide fluid connection to a separate photobioreactorpanel unit.

In a preferred embodiment, the photobioreactor panel unit furthercomprises one or more baffles extending between the first and secondsidewalls, so that the interior comprises a plurality of compartments.

In another embodiment, the invention provides photobioreactor modules,comprising two or more photobioreactor panel units of the second aspectof the invention, wherein the container of each photobioreactor is influid communication with all other containers in the photobioreactormodule.

In a third aspect, the present invention provides methods for making thephotobioreactors of the first or second aspect of the invention,comprising assembling the photobioreactor of the first or second aspectof the invention from their component parts.

In a fourth aspect, the present invention provides methods for algalgrowth, comprising incubating algae in a growth medium in aphotobioreactor of the first or second aspect of the invention, andexposing the algae to light. In a preferred embodiment, the methodfurther comprises harvesting the algal cells. In a further preferredembodiment, the method comprises isolating biological products from theharvested algal cells. In a further embodiment, the methods for growingalgae comprise incubating the algae in the presence of a nutrient sourceselected from the group consisting of wastewater from concentratedanimal feeding operations, agriculture runoff water, underground salinewater, industrial wastewater, domestic wastewater, contaminatedgroundwater, waste gases emitted from power generators, and flue gasemissions from fossil fuel fired power plants.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. An exemplary geometric configuration of the reactor. Cleartransparent materials can be glass, or rigid/flexible plastic (e.g.,polycarbonates, PVC, acrylic, polyethylene).

FIG. 2. A reactor unit with multiple compartments partially separated bybaffles.

FIG. 3. Examples of configuration of single reactor unit with a linearand serpentine-shape are presented.

FIG. 4. Another geometric configuration of the reactor, reactor unitscan be made using plastic extrusion technology. A reactor unit consistsof three parts: body panel, and top- and base-caps.

FIG. 5. Another geometric configuration of the reactor consisting ofmultiple reactor units. Culture suspension in individual units isconnected through both top- and base-cap structures.

FIG. 6. Examples of single reactor unit with various lengths of lightpath (P1, P2, P3, and Px) represent different lengths of light paths ofreactors.

FIG. 7. Examples of reactor modules with various lengths of light pathsof reactors.

FIG. 8. Aeration system. Pure CO₂ or CO₂-rich flue gas can be blendedwith air at a certain ratio to facilitate culture mixing while providingcarbon source for algal photosynthesis.

FIG. 9. Evaporative cooling option. The reactor can be maintained atoptimal culture temperatures during the summer or when temperature isabove the optimal temperature range by evaporative cooling.

FIG. 10. Internal temperature control option. An optimal culturetemperature can be also maintained in the reactor by an internaltemperature control system. Metal tubing is inserted into reactor units,in which warm or cold water is circulating to effect culturetemperature. Waste heat from power generator burning algal residue oralgae-derived biogas can be used to maintain culture temperature duringthe winter season, or whenever a higher temperature is desirable.

FIG. 11. A computer-based monitoring system is integrated into reactorunits and/or modules to monitor and regulate culture pH, temperature,NO₃ ⁻/PO₄ ⁻³ levels, and O₂ and CO₂ concentrations. As an integratedcomponent of algal harvesting system, optical-density sensors areinserted into selected reactor units in a reactor module for on-linemonitoring of algal cell density and that, in turn, will be used tocontrol algal harvesting.

FIG. 12. Inclination of a reactor relative to sun's radiation.

FIG. 13. Reactor units and/or reactor modules can be set up at the samelevel, or at different levels. In this particular case, the reactorunits are set-up in a step-wise fashion.

FIG. 14. Schematic diagram of an indoor reactor. Reactor sections insolid lines represent a single unit. Sections with dash-lines indicatepotential extension of individual reactor unit to any desired length.Individual reactor units can be connected in cascade and direction ofculture flow from larger light-path units to narrower light-path units,or visa versa.

FIG. 15. A hybrid open pond-closed reactor systems. While open pondsserve multi-functions a) as a waste holding pond; b) initial algaladaptation to the field environment; and c) initial nutrient removal andbiomass production stages, the closed reactor will i) provide seedculture to the open raceway; ii) polish wastewater to completely removenutrients from the wastewater; iii) enhance biomass production; and iv)induce cellular accumulation of desirable products (such as high-valuepigments, lipids/oil, proteins, or polysaccharides).

FIG. 16. Examples of reactor top covers. It is preferred to betransparent materials, such as glass or clear plastic.

FIG. 17. Top- (A) and side-views (B) of end-strut. The end-strut can bemade of, for example, stainless steel or plastic sheet.

FIG. 18. Top- (A) and side-views (B) of corner-struts. The comer-strutscan be made of, for example, metal or high-strength plastic materials.

FIG. 19. Configurations of support struts. Struts can be made of, forexample, metal, plastic, or concrete. Sidewalls can bound directly tothe struts or to a thin strip of, for example, metal or plastic sheet.In the latter case, the strut only serves a supporting function.

FIG. 20. Top- (A) and side-views (B) of baffle-mounting strut. Thebaffle can be made of, for example, metal, glass, or high-strengthplastic sheet.

FIG. 21. Top- and side-views of reactor sections where the length oflight path changes as desired. The Z-shape connectors can be made of,for example, a plastic sheet (such as PVC) or a metal sheet (such asstainless steel). Such design enables a single reactor unit to havedifferent lengths of light path, as desired.

FIG. 22. Photobioreactor units an/or photobioreactor modules can be setup at the same level on the ground. Manifold systems are set for supplyof growth medium/algae and harvesting and/or draining of algal culturefrom reactor units/modules.

DESCRIPTION OF THE INVENTION

In a first aspect, the present invention provides a photobioreactorcomprising:

a container adapted for holding fluid, comprising

-   -   opposing first and second sidewalls, wherein at least one of the        first and second sidewalls is transparent;    -   opposing first and second endwalls;    -   a container bottom; and    -   a container cover,    -   wherein the first and second sidewalls comprise a plurality of        separate sections, and wherein the separate sections are in        fluid communication;

support struts for connecting the plurality of separate sections of thefirst and second sidewalls;

at least one inlet port in fluid communication with the container;

at least one outlet port in fluid communication with container;

an aeration system in fluid communication with the container; and

a temperature control system connected to the container so as to controltemperature of fluid within the container.

The photobioreactor of this first aspect of the invention can beexpanded as desired through the use of the sidewall sections and struts,to produce an industrially useful flat panel photobioreactor, asdescribed in more detail below, fulfilling a great need in the art.

The photobioreactors of the present invention, and modules comprising aplurality of such photobioreactors, are designed to sustain highperformance of mass cultivation of algae, particularly using solarradiation, for commercial production of, for example, renewable biofuelsand other value-added products, as well as for bioremediation of, forexample, wastestreams of industrial, agriculture, and domestic origin.

As used herein, the term “in fluid communication with” means aconnection that permits the passage of liquids or gases between therecited components.

A schematic drawing of an exemplary photobioreactor of this first aspectof the invention is shown in FIG. 1.

At least one of the first and second sidewalls of the container istransparent. In embodiments where only one of the sidewalls istransparent, that is the sidewall that will face the sun when thephotobioreactor is used for algal growth outdoors. Transparent sidewallsof the container can be made of any transparent material including, butnot limited to, glass or plastic plates. Plastic sidewalls can be madeof materials including, but not limited to, PVC, polycarbonate, acrylic,or polyethylene. Non-transparent sidewalls can be made of any suitablematerial, including but not limited to plastic (such as PVC,polycarbonate, acrylic, or polyethylene), fiberglass, stainless steel,concrete, and plastic liners. Such non-transparent sidewalls can be usedas the sidewall facing away from the sun when the photobioreactor isused for algal growth outdoors.

As used herein, the term “plurality” means two or more; thus, thesidewalls comprise at least two separate sections. The transparentsidewall that will face the sun (“front sidewall”) comprises a pluralityof sections, while the “back sidewall” (which can be transparent ornon-transparent) can be a single section along the length of thephotobioreactor, or can also comprise a plurality of sections.

The distance between the inner sides of the two sidewalls is the “lightpath,” which affects sustainable algal concentration, photosyntheticefficiency, and biomass productivity. The light path can be betweenapproximately 5 millimeters and 40 centimeters; preferably between 100millimeters and 30 centimeters, more preferably between 50 millimetersand 20 centimeters, even more preferably between 1 centimeter and 15centimeters, and most preferably between 2 centimeters and 10centimeters. The most optimal light path for a given application willdepend, at least in part, on factors including the specific algalspecies/strains to be grown and/or specific desired product/s to beproduced.

The height of reactor sidewalls can range from 2 feet to 8 feet or more,depending on the type and thickness of sidewall materials used andeconomic considerations. For example, the higher the sidewall, thethicker the wall material required, and the higher the material cost.Alternatively, while low sidewalls save on material costs, an increasednumber of separate sidewall sections are required to meet a given totalvolume requirement; in addition, an increased number and/or more complexsystem of inlet ports, outlet ports, aeration systems, and temperaturecontrol systems would be required. So, for each individual application,the height of the sidewall is optimized from both engineering andeconomic perspectives.

In one embodiment, the sidewalls and endwalls are substantially flat;this embodiment is preferred when using glass, rigid plastic sheets,stainless steel or concrete to construct the sidewalls and endwalls. Inother embodiments, particularly where the sidewalls and/or endwalls aremade of flexible plastic sheets, which are supported by external struts,the sidewalls may have some curvature imposed by individual struts.

The photobioreactor of this first aspect of the invention can range inlength from one meter to hundreds of meters. Due to the design featuresof the present photobioreactor, the sidewalls can be of any length.

The heights of the face (front) sidewall and back sidewall can be thesame or slightly different. For example, the front sidewall can beslightly shorter than the back sidewall in embodiments where thephotobioreactor is designed to incline to a certain angle toward the sun(for maximizing solar energy harvesting for algal photosynthesis; seebelow).

The front sidewall can be made of the same or different clear materialsfrom the back sidewall, with optimization based on cost-saving, maximumlight penetration capacity, and heat mass transfer efficiency. Forexample, in a cold weather region, reactor sidewalls made of plastic mayprovide an advantage over glass plates because the plastic material mayreduce heat loss from culture suspension to the surroundings.Alternatively, glass has a higher heat transfer efficiency, and thus theuse of glass sheets as the front and/or back sidewalls will allow thereactor to dissipate access solar-heat more efficiently than a plasticsidewall material in a warmer climate region.

In another example, if one clear material is more transparent and/orable to maintain high transparency longer than another type of material,but the latter is less expensive, then the higher quality material maybe used for front sidewall whereas the cheaper material may be used forthe back sidewall. Further cost reduction can be obtained by using backsidewalls that are not transparent and are made using lower cost and/orhigher strength material, such as concrete, stainless steel, plasticliners placed on the top of an earth bank (“berm”) of desired shape, andfiberglass.

In various preferred embodiments, the first and second sidewallscomprise 2, 3, 4, 5, 10, 25, 50, 75, 100, 150, 200, 500, or moreseparate sections. In these various embodiments, the front sidewall cancomprise 2, 3, 4, 5, 10, 25, 50, 75, 100, 150, 200, 500, or moreseparate sections while the back sidewall can comprise a single section,or can also comprise 2, 3, 4, 5, 10, 25, 50, 75, 100, 150, 200, 500, ormore separate sections. There is no upper limit on the number ofseparate sections that can be used, so long as appropriate struts areused. In a preferred embodiment, the number of support struts in thephotobioreactor is n+1, wherein “n” is the number of sections of thefirst and/or second sidewalls.

The container bottom can be made of any material that can form a sealwith the sidewalls to define an interior space of the container that canhold and retain an algal culture to be grown in the photobioreactor.Such materials include but are not limited to concrete, tile, glass, orthin sheet of plastic or stainless steel. There is no requirement thatthe container bottom be transparent, although it may be transparent. Ina preferred embodiment, the container bottom is made of the samematerials as one or both of the sidewalls and/or endwalls.

The photobioreactor also comprises a container top to preventairborne-dusts/microbial organisms from entering the culture, and toprevent water evaporation. The top can be made of glass or plasticmaterials, and is preferred to be transparent to allow lightpenetration. (See, for example, FIG. 16) In a most preferred embodiment,the top is made of the same materials as the one or both of thesidewalls and/or endwalls. It is also preferred that the top be set atan angle to the horizon or has curvature. A cover with a tilted angle orcurved shape will prevent accumulation of water drops with or withoutalgal cells on the inner surface of the cover. Potential build up ofwater drops and/or growing algal cells on the inner surface of the coverwill reduce light penetration into the culture. The cover is thuspreferably designed such that it is easy to remove and reset to enablecleaning of reactor inner surface, as needed.

In this first aspect of the invention, support struts are used toprovide a framework to connect the separate sections of the first and/orsecond sidewalls. The struts can be of any material suitable for thispurpose, including but not limited to metal and high strength plastic,concrete, or ceramic. As discussed above, the struts can serve a supportfunction, and also provide the surface area for sidewalls to join.Struts may also define the length of reactor light path (depth) andconfiguration of reactor unit (e.g., linear or serpentine shape). Ingeneral, the struts provide support and surface area for sidewalls tojoin or to have the sidewalls bonded onto the strut. The inner structureformed by sidewalls join one another and by the front and back sidewallscreates a single interior space in the container, which can be brokeninto compartments through the use of baffles, described more fullybelow.

Struts are thus the means to join sidewalls piece by piece, side byside, which enable a single photobioreactor unit to have any length orconfiguration. Some struts may also provide frame structures for bafflesto fix on them (see below) to create many inner-compartments within asingle photobioreactor unit. Struts can also be used to provide supportfor endwalls in either linear or serpentine configurations, as well ascorners (See FIGS. 17-18 for exemplary depictions of struts used tosupport endwalls or corners.)

In a preferred embodiment, the support struts are located external tothe container and are arranged to provide support to hold the containerand also to provide the surface for individual sidewalls to join.

In a preferred embodiment, for a given umber (n) of sidewalls, thenumber of struts including end-struts is ‘n+1’.

There are a number of design options for providing support struts toconnect the plurality of separate sections of the first and/or secondsidewalls, including but not limited to:

The strut not only provides the surface area for the edges of twosidewalls to be bonded to it, but also to provide weight support fromthe external direction. In this embodiment, it is preferred that thesurface of the strut is smooth and flat, and also compatible with thetype of bonding agent (including but not limited to glue, silicone,epoxy, etc) used to join the sidewall material to the strut. See, forexample, FIG. 19.

The strut serves as support and also provides a surface area for theedges of two sidewalls to contact. However, the sidewall is not bondeddirectly to the strut, but to a thin sheet of plastic or metal (e.g., astrip of thin plastic or stainless steel sheet of the same size of thestrut's inner surface) inserted between the sidewalls and the strut.Since the strut is not directly contacted and bonded to the sidewalls,the nature of the strut material and/or quality of the inner surface ofthe strut becomes less critical. Also, in case a sidewall needs to bereplaced, it can be easily removed from the strut structure. Thesidewalls bonded to a high quality, thin plastic, or stainless steelstrip ensures better sealing at the sidewall joining region. See, forexample, FIG. 19.

A gasket (any suitable material, including rubber, plastic, or analogousmaterials) can be placed between the sidewall and inner surface of thestrut or inner surface of a thin plastic or metal strip to prevent orreduce thermo-contraction/expansion due to temperature changes underoutdoor conditions. For the same reason, the gasket will also be placedbetween the edge of the sidewall and base cap or bottom base of thereactor.

The type of bonding agent used depends on the type of materials used insidewall construction and the strut design, as discussed above. Those ofskill in the art will be able to optimize the type of bonding agentbased on the teachings herein. Non-limiting examples of such bondingagents include, but are not limited to, various glues, epoxies, andsilicone.

It will be apparent to those of skill in the art that many other suchdesigns for strut support of the container sections could beimplemented, based on the teachings herein.

The photobioreactors contain at least one inlet port in fluidcommunication with the container, for introducing fluid, including butnot limited to culture medium, algal suspensions, water/wastewater, andnutrient solutions, into the container. In a preferred embodiment, theinlet port is located at an endwall of the photobioreactor, preferablynear the top of the container. There can be one or more inlet portslocated at or near the endwall to deliver culture medium, algalsuspensions, water/wastewater, or nutrient solutions. Differentsolutions can also enter the reactor through a single inlet. The purposeof inlet/s located at or near the endwall is to create a nutrientgradient in which the highest nutrient concentration is near the inletand the lowest concentration is at the far end of the reactor. Differentnutrient concentrations affect the growth and biochemical composition ofalgal cells. For example, nutrient-rich medium may stimulate and sustaina high growth rate and biomass productivity, whereas nutrient depletedmedium may stimulate biosynthesis and cellular accumulation of neutrallipids, long chain fatty acids, and/or secondary carotenoids. A nutrientgradient created in a reactor of this design thus allows a continuousshifting of algae from a high biomass production mode to a highaccumulation of specific desired product mode. The outlet at or near thefar end of the reactor will ensure harvesting algal cells of the highestcontent of desired compound/s/product/s in the cell.

Alternatively, multiple inlets ports may be located at certain distanceapart from one another to ensure nutrient and/or algal cellconcentrations to be more or less homogenous throughout the reactorunit. In this case, the cells grown in a given reactor unit or modulewill have identical, desirable physiological status for specificapplications. In the case of a linear reactor unit, multiple inlet portsmay be located along the reactor length, when desired as discussedabove. The distance between inlets ports can be optimized for a givenuse.

The inlet port(s) can be located at any height relative to the sidewallsor struts. When multiple photobioreactor units are arranged in cascadeand culture suspension flows from one reactor unit to next one at thedownstream site, it is preferred to have the inlet port(s) near the topof the photobioreactor.

The photobioreactors of this first aspect of the invention comprise atleast one outlet port in fluid communication with the container, forremoving fluid from the container, including algal culture suspensionsfor harvesting. In a preferred embodiment, the outlet port is located atan opposite endwall of the photobioreactor from the inlet port.Placement of the outlet port is based on the specific needs of the user.For example, in using the overflow principle as a means of harvestingand culture level control, the outlet port is preferably placed near thetop of the photobioreactor or the position where an ideal culture levelcan be maintained. In this embodiment, it is preferred to have theoutlet port located at or near the far endwall (away from the inletport). Alternatively, if harvesting is controlled by, for example, asolenoid valve-mediated outlet, then the outlet port is preferablyplaced near the bottom of the reactor. In this embodiment, the outletport is designed such that it serves both for harvesting and reactordrainage. When the outlet port(s) is/are located near the top of thephotobioreactor, a separate outlet near or at the bottom of the endwallis necessary for reactor drainage.

For a single photobioreactor, it may not be critical to have the outletat a particular location relative to the endwall. However, when multiplephotobioreactors are to be arranged in parallel as a module,capital/maintenance costs of piping/connecting materials may be reducedif all the outlet ports are located at the endwalls and connected to acommon manifold piping system (FIG. 22).

The photobioreactors of this first aspect of the invention also comprisean aeration system in fluid communication with the container. Theaeration system comprises any suitable system for (a) introducing acarbon dioxide supply into the container; and (b) introducing compressedair to effect culture mixing. Such an aeration system may comprise, forexample, air tubing made of flexible, or rigid plastic (such as siliconor PVC tubing), or metal (such as stainless steel). For example,aeration can be provided by compressed air passing though perforatedtubing running along the container bottom (FIG. 8). In case that reactorsidewalls are quite high (for example, 6 feet or higher), a secondaeration line may be introduced, for example, halfway between the topand bottom of the reactor to enhance culture mixing. Holes of certaindiameter (preferably between 0.˜12.0 mm) set certain distance(preferably 10˜50 mm) (from one another along the tubing provide airbubbles to effect culture mixing. Carbon dioxide can be blended withcompressed air at a certain percentage (preferably from 0.1% up to 20%of CO₂) to provide carbon source for algal photosynthesis. In somecases, organic carbon (for example, in the form of acetic acid and/orglucose) can be added as needed into the culture medium to support algalgrowth. Any suitable source of carbon dioxide can be used, including butnot limited to industrial grade, food grade, CO₂-rich flue gases emittedfrom power generators burning coal, biomass (including algal biomassand/or biomass residues after high-value products are extracted),natural gas, biogas (e.g., ethanol, methane obtained from anaerobicdigestion/fermentation of algal biomass or biomass residues and/or fromanaerobic digestion of wastewater), and liquid fossil fuel or biofuels(including algae-based biodiesel).

The photobioreactors of the first and second aspects of the inventioncan be linear or serpentine in shape; see, for example, FIG. 3.

In a further embodiment of this first aspect, the photobioreactorfurther comprises at least one drainage outlet. In a preferredembodiment, a drainage outlet is located in an endwall opposite theinlet port, and near the bottom of the endwall. As discussed above, whenan over-flow mode is used for harvesting, then, the drainage has aseparate port, preferably close or at the bottom of the reactor tofacilitate draining of water/culture with minimum energy requirement.Normally, there will be a single drainage outlet per reactor unit.However, for a reactor unit of extended length, multiple drainageoutlets may be preferred. For instance, for a serpentine—shape reactorunit, drainage ports may be located at individual endwall sides.

In a further embodiment of this first aspect, the container furthercomprises one or more baffles connected to the first and secondsidewalls, so as to form a barrier and to partially separate thecontainer into multiple compartments. Baffles are especially preferredin embodiments where the sidewalls are long (for example, 10 to 1,000meters long) to partially separate the container into multiplecompartments. The baffles are designed such that they allow the upperportion of individual compartments to open to one another. While thebaffle can be of any height desired by a user, it is preferred that thebaffle height is between 60% and 90%, and more preferably between 70%and 80% of the height of the sidewalls. When such baffles are used,culture suspension in individual compartments can flow from onecompartment to another, although the lower parts of individualcompartments are isolated (FIG. 2). The purpose of such design is thatin case one particular compartment is broken, the rest of the containercompartments will still hold most of the culture suspension.

The baffle can be made of any material suitable for inclusion in acontainer supporting algal growth, including but not limited to glass,stainless steel plate, and rigid plastic sheet. In general, thematerials used for one or both sidewalls and/or end walls can be usedfor baffle structure as well. The thickness of the baffle depends on thetype and mechanical strength of materials, and height of the baffle. Fora strut where a baffle is joined, the strut provides not only thesurface for sidewalls to join, but also the surface on which the baffleis mounted. Both the side- and lower-edge of the baffle can be joined tothe strut frame to create separation of individual compartments at thelower section, leaving the upper section open to enable culturesuspensions in individual compartments to communicate and mix. In oneembodiment, a solenoid-controlled valve is located at or near the bottomof the baffle, which can open or close as needed (See FIG. 20). When thevalve is open, all the compartments within a reactor unit areinter-connected not only via the upper section, but through the valveopening. When the valve is closed, only the upper parts of individualcompartments are inter-connected. Under normal conditions, the solenoidvalve on the baffle is in the open position. It can be closed, forexample, when an accident occurs, (e.g., container leaking, sidewallsbroken), or as otherwise desired by the operator.

In one non-limiting example of a photobioreactor according to this firstaspect of the invention, a single linear photobioreactor unit is 100meters long. Nine baffles are inserted equally apart (10 metersdistance) from one another to create 10 compartments in the container.The height of the baffles is somewhat shorter than that of sidewalls orstruts, cell culture suspensions in all individual compartments aremixed on the top part. The purpose of the baffle structure in thereactor unit is to prevent loss of the entire culture in case one pieceof sidewall is broken due to whatever reasons. The amount of culturelost in this case is only from the broken compartment along with a smallportion of culture above the height of the baffle structure. It issuggested that an appropriate distance between individual baffles wouldbe 10 to 50 meters. In other words, the reactor compartment createdbetween two baffles should provide an inner space of 5,000 to 100,000liters.

In another embodiment, the invention provides photobioreactor modules,comprising two or more photobioreactors of the first aspect of theinvention, wherein the containers of the photobioreactors are in fluidcommunication. In this embodiment, the individual photobioreactorcontainers may in direct fluid communication at the inlet (ie: an inletsupplying culture medium, etc. to each of the photobioreactor containersin the module; they may be in fluid communication when set in parallelthe fluid from individual photobioreactor containers can be harvestedthrough outlet ports connected to a common harvesting/draining manifoldsystem; a combination thereof, or via some other means of fluidcommunication between the individual photobioreactor containers..

The two or more photobioreactors in a module may have the same ofdifferent light paths. The invention also provides photobioreactorclusters, comprising two or more photobioreactor modules of theinvention. The two or more photobioreactor modules in a cluster may alsohave the same or different light paths. FIG. 7 illustrates an example ofa particular reactor cluster in which individual reactor modules vary inthe length of the light path. In this case, harvesting of algalsuspension can take place at the end of each individual reactor module,or at the end of a cascading, multiple reactor cluster. FIG. 13 shows anexample of individual photobioreactors set stepwise so that algalculture can flow down by gravity from the top photobioreactor to thephotobioreactor of the lowest level. All photobioreactors, modulesthereof, or clusters thereof can also be set up at the same level. FIG.3 is a case where an individual photobioreactor is set at the samelevel.

A photobioreactor “unit” is as described for the first aspect of theinvention; ie: a single photobioreactor. It can have the same ordifferent lengths of light path along its length, and can be linear orserpentine in shape, and usually it has one or multiple inlets at oneend of the reactor, and one or multiple outlets located at the oppositeside of the reactor. See FIG. 21 for an example of design to achieve adifferent light path in a photobioreactor.

The “photobioreactor module” comprises multiple photobioreactor unitsarranged in parallel. When the reactor units are in a linearconfiguration, all the inlets of all individual reactor units face onedirection, whereas outlets of reactor units face an opposite direction.If reactor units are in a serpentine configuration, all the inlets andoutlets may be either at the same or in an opposite direction. In apreferred embodiment, algal cells of the same physiological or nutrientstatus are maintained within a reactor module.

A “photobioreactor cluster” comprises multiple reactor modules.Individual modules may have the same or different lengths of the lightpath. Preferably, a culture flows from reactor modules having thelargest reactor light path and/or highest nutrient load growth medium toreactor modules of reduced light path to reactor modules of thenarrowest light path and nutrient depleted growth medium. The connectionof individual reactor modules is flexible, depending on specific algalspecies/strain and/or production of specific products. For example, areactor cluster comprising reactor units/modules of large light pathwill be preferred for production of the high value phycobiliproteins orlong-chain polyunsaturated fatty acids from certain cyanobacteria andmicroalgae. In contrast, to maximize production of neutral lipids andsecondary carotenoids, the reactor cluster preferably comprises multiplereactor modules arranged such that the culture flows from reactormodules of large light paths to reactor modules of narrower light paths.

In a second aspect, the present invention provides a photobioreactorpanel unit, comprising:

a container adapted for holding fluid, comprising opposing first andsecond sidewalls and opposing first and second endwalls, wherein thecontainer define an interior, a top opening and a bottom opening,wherein at least one of the first and second sidewalls is transparent,and wherein the first and second sidewalls are substantially flat;

a top cap that fits over the top opening of the panel body; and

a base cap that fits under the bottom opening of the panel body;

one or both of the top cap and the bottom cap further comprise one ormore channels, to provide fluid connection to a separate photobioreactorpanel unit.

The photobioreactor panel unit of this second aspect of the invention isa single unit that can be connected together via the channels to form amulti-unit photobioreactor module, as described below. As such, itprovides for scale up of flat plat-type design to a commercial level,and thus fulfills a great need in the art.

In a preferred embodiment of this second aspect, the container, the basecap, and the top cap are plastic, and the photobioreactor panel unit ismade using plastic extrusion technology, where the plastic is pushedand/or drawn through a die to create long objects with a fixedcross-section. Hollow sections can be formed, for example, by placing apin or mandrel in the die. Extrusion may be continuous to produceindefinitely long materias, or semi-continuous, to repeatedly producemultiple shorter pieces.

The advantage of plastic extrusion technology is to enable massproduction of standard reactor units at considerably reduced price. Thestandard sizes of reactor units will also enable simple, quickassembling of reactor units into modules or arrays. This method willalso increase the height of the reactors potentially to 10 to 20 metersin height, thereby reducing the number of reactor units and thusreducing captical/maintenance costs.

The first and second sidewalls of this second aspect of the invention,and various embodiments thereof, are similar to those described in thefirst aspect of the invention.

The top-cap fits over the top opening of the panel body to preventairborne-dusts/microbial organisms from entering the culture, preventwater evaporation, release air and excess oxygen generated through algalphotosynthesis, and to connect individual panels one another side byside. For example, the top cap shown in FIG. 5 actually comprisesmultiple individual top caps joined to one another. Both base and topcaps are designed such that they can be readily connected for addedlength as desired. Thus, algal culture suspensions in individualflat-panel units can mix and flow from one panel to another (FIG. 5).The bottom cap fits under the bottom opening of the panel body, toprovide integrity to the panel body for holding fluid, and thus thecapability to serve as an algal photobioreactor.

Embodiments of the top cap and the bottom cap are similar to thosedescribed above for the container bottom and top.

In this second aspect, one or both of the top cap and the bottom capfurther comprise one or more channels, to provide fluid connection to aseparate photobioreactor panel unit, to allow culture suspension to flowfrom one individual panel to another. In a preferred embodiment, the oneor more channels are present in the top cap.

In a preferred embodiment of this second aspect, the interior comprisesone or more baffles extending between the first and second sidewalls, sothat the interior comprises a plurality of compartments. The distancebetween baffles can be variable based on a user's specific needs, asshown in FIG. 4. Baffles and embodiments thereof are as described abovefor the first aspect of the invention.

In another preferred embodiment, the base cap further comprises anaeration system, as disclosed above for the first aspect of theinvention. In a preferred embodiment, the aeration system comprisesair-bubble tubing inserted along the bottom of the base cap to provideaeration to affect culture mixing and provide carbon dioxide supply.Aeration systems and embodiments thereof are as described above for thefirst aspect of the invention.

In another preferred embodiment, one or both of the top cap and bottomcap further comprise an inlet port in fluid communication with theinterior, for introducing fluid, including but not limited to culturemedium, algal suspensions, and nutrient solutions, into the interior ofthe photobioreactor panel unit. In a preferred embodiment, the inletport is located in the top cap. It will be understood that, wheremultiple photobioreactor panel units of this second aspect areconnected, that only one such inlet port is required, and thus not allindividual units need to comprise such a port. However, any or all ofthe individual units may comprise an inlet port. Inlet ports andpreferred embodiments thereof are as described above for the firstaspect of the invention.

In another preferred embodiment of this second aspect, one or both ofthe top cap and bottom cap further comprise at least one outlet port influid communication with the interior, for removing fluid from thecontainer, including algal culture suspensions for harvesting. In apreferred embodiment, the outlet port is located in the top cap. It willbe understood that, where multiple photobioreactor panel of this secondaspect are connected, that only one such inlet port is required, andthus not all individual units need to comprise such a port. However, anyor all of the individual units may comprise an inlet port. Outlet portsand preferred embodiments thereof are as described above for the firstaspect of the invention.

In a further embodiment of this first aspect, at least one of the topcap and bottom cap further comprise at least one drainage outlet. In apreferred embodiment, a drainage outlet is located in the bottom cap.Drainage ports and preferred embodiments thereof are as described abovefor the first aspect of the invention.

The following embodiments apply to either or both of the first andsecond aspects of the invention. In one further embodiment, the distancebetween the first and second sidewalls varies at different locations inthe photobioreactor. As discussed above, the distance between the innerface of the first and second sidewalls constitutes the light path of thephotobioreactor. In this embodiment, the photobioreactor can have anequal length of either long or narrow light paths, or vary in light pathalong the horizontal axis, i.e., having a long light path at one end,and gradually reducing the length of light path while moving towards theopposite end of the photobioreactor. An example of a photobioreactorwith different distances between the first and second side walls (ie:different light paths) is shown in FIG. 6.

Such variable light paths can be used, for example, with specific algalstrains to be cultured and/or specific desirable end-products (e.g.,high-value carotenoids, total lipids, and proteins) to be produced atdifferent locations within the photobioreactor, as discussed above andas is known in the art (Hu et al., Biotechnology and Bioengineering 51:51-60 (1996); Hu et al., European Journal of Phycology 33: 165-171(1998). In one non-limiting example, a photobioreactor has a light pathof 100 mm at one end and a light path of 20 cm at the other end; anytype of desirable variation can be employed, as will be apparent tothose of skill in the art based on the teachings herein.

In a further embodiment of the first or second aspect, thephotobioreactor further comprises a temperature control system. Anysuitable temperature control system can be used. One exemplary approachis the application of external temperature control (FIG. 9). Water of acertain temperature from near the top of one side of the reactor canflow down against the external surface (ie: the outside face of one orboth of the sidewalls) to maintain a desirable internal temperaturelevel for algal growth or specific cellular metabolism. In someembodiments, only the sidewall facing the sun is cooled. For example,one or more water pipes can be placed at or near the bottom ofsun-facing side of the reactor. A series of water-spray heads can befixed at a certain angle to the water pipe, from which tempered watercan be sprayed onto the sun-facing surface of the reactor. In apreferred embodiment, the photobioreactor with this type of temperaturecontrol system would further comprise a water tray underneath the basecap or container bottom, to collect the cooling water. In a furtherpreferred embodiment, the external temperature control system comprisesa pipe that collects water from a reservoir; the water tray subsequentlyreturns the water to the reservoir, where the water temperature can beeither increased or decreased as necessary for water recycling.

An alternative temperature control system comprises internal temperaturecontrol, in which tubing, preferably metal tubing (since metal materialsin general have higher heat transfer efficiency than plastic or glass),preferably running along the bottom of the photobioreactor through whichtemperature-regulated water is circulated to maintain the culturetemperature at a desirable level (FIG. 10).

In a further embodiment, the temperature control system comprises atemperature controlled compressed air source. The culture suspension isaerated with temperature-controlled compressed air. In this embodiment,a temperature control element is embedded into the air container to warmor cool compressed air. The cooling element can be a refrigeration unit,an evaporative cooling unit, or equivalents thereof. The heating elementcan be a commercially available product, or a unit that can utilizewaste heat generated from power generators.

In various further embodiments of the first and second aspects of theinvention, the photobioreactor further comprises systems and sensors forcontrol of culture pH, NO₃ ⁻/PO₄ ³⁻ levels, and O₂ and CO₂concentrations. For example, a monitoring system can be acquired fromAquatic Eco-System, Inc. (Apopka, Fla. 32703).

Any of these control systems and sensors can be implemented using anautomatic control system and methodology. In a preferred embodiment, acomputer-based monitoring system is integrated into the photobioreactorand/or modules thereof, to monitor and regulate culture pH, temperature,NO₃ ⁻/PO₄ ³⁻ levels, O₂ and CO₂ concentrations. As an integratedcomponent of algal harvesting system, optical-density sensors can alsobe inserted into selected photobioreactor units in a module for on-linemonitoring of algal cell density, which in turn will be used to controlalgal harvesting (FIG. 11).

In a further embodiment of the first and second aspects of theinvention, the photobioreactor further comprises a means to effectinclination of the photobioreactor. A direct relationship between solarenergy and productivity is observed: the higher the amount of solarenergy that is admitted by varying the reactor tilt angle according toseason, the higher the productivity sustained in the culture. Thereactor tilt angle exerts a significant effect on the optimal populationdensity and thus on the productivity of cell mass, due to its effect onthe amount of solar radiation impinging on the surface of the reactor.

For a given seasonal temperature, biomass productivity depends on theamount of solar radiation admitted to the photobioreactor. Generally,for an equator-facing, one-sided photobioreactor, the optimal tilt anglefor maximal year-round energy collection is a tilt angle approximatelyequal to geographic latitude. The effect of photobioreactor orientationbecomes more significant the higher the geographic latitude since theavailability of sunlight is more limited than it is closer to theequator. Small tilt angles of 10° to 30° in summer and larger angles inthe vicinity of 60° in winter result in maximal productivities for theseseasons (FIG. 12).

A benefit in orientating and tilting reactors at various angles to thesun both daily and throughout the year is to reap the maximal potentialassociated with the biological conversion of solar energy. Studying theeffect of the frequency of optimizing the tilt angle on overall annualproductivity shows that frequent adjustment for the optimal reactorangle throughout the year will result in the highest overall annualproductivity. A potential benefit in orientating and tiltingphotobioreactors at various appropriate angles to the sun on a seasonalbasis: up to 40% enhancement in annual biomass yield can be achievable.From a practical standpoint, adjusting the tilt angle twice a year wouldsignificantly enhance overall productivity. This is particularly truefor production sites located at higher latitudes.

Any means for effecting inclination of the photobioreactor can be used,such as those disclosed in Hu et al. Journal of Fermentation andBiotechnology 85: 230-236 (1998). In another embodiment, thephotobioreactors of the invention can be placed on a berm, such as amound or bank of earth, that is raised and sized to provide theappropriate inclination of the photobioreactor relative to the sun. Inthis embodiment, the sidewall facing the berm does not have to betransparent. In this embodiment, the berm may also serve to insulate andhelp to maintain suitable culture temperatures in the photobioreactor.

To a large range of geographic latitudes, the photobioreactors should beplaced in a south-facing orientation to receive maximum solar radiation.However, for certain geographic latitudes (such as Israel), verticaleast/west-facing photobioreactors can receive more solar input duringsummer months than even the most optimally tilted south-facing reactors.Systematic use of our equation enables the optimal configuration ofreactors to be identified for any geographic latitude, permitting therelative benefits of east-west orientation to be assessed.

In certain embodiments, the photobioreactors can be used for indooralgal cultures. In this case, artificial illumination can be provided,and one particular source of light is fluorescence tubes (FIG. 14) Theonly difference between indoor and outdoor reactors of this design isthe source of light. The reactors themselves can be identical.

In a further embodiment, the photobioreactor of the first and/or secondaspect of the invention can be used in combination with an open pondsystem (FIG. 15). While the open pond will serve multi-functions a) as awaste holding pond; b) initial algal adaptation to the fieldenvironment; and c) initial nutrient removal and biomass productionstages, the closed photobioreactors of the invention can be used to i)provide seed culture to the open raceway; ii) polish wastewater tocompletely remove nutrients from the wastewater; iii) enhance biomassproduction; and iv) induce cellular accumulation of desirable products(such as high-value pigments, lipids/oil, proteins, or polysaccharides).

The photobioreactor of the first or second aspect of the invention canbe made as complete individual reactors or modules thereof in amanufacturing site and shipped to a reactor application site forinstallation and operation, can be made and shipped as separate parts tothe application site for assembly and operation, or can be made on-sitenear or within the reactor application site.

Thus, in a third aspect, the present invention provides methods formaking the photobioreactors of the first or second aspect of theinvention, comprising assembling the photobioreactor of the first orsecond aspect of the invention from their component parts. Suchcomponent parts will be apparent to those of skill in the art, based onthe discussion above of the first and second aspects of the invention.

In a first embodiment, the method comprises assembling a photobioreactorby assembling a container adapted for holding fluid with support struts,wherein the container comprises:

opposing first and second sidewalls, wherein at least one of the firstand second sidewalls is transparent;

opposing first and second endwalls;

a container bottom; and

a container cover,

wherein the first and second sidewalls comprise a plurality of separatesections, and wherein the separate sections are in fluid communication;and

wherein the support struts are assembled to connect the plurality ofseparate sections of the first and second sidewalls.

In exemplary preferred embodiments of this first embodiment, thephotobioreactor further comprises at least one inlet port in fluidcommunication with the container; at least one outlet port in fluidcommunication with container; an aeration system in fluid communicationwith the container; and a temperature control system connected to thecontainer so as to control temperature of fluid within the container.The methods can further comprise adding other embodiments of thephotobioreactors as described above in the first and second embodimentsof the invention.

In a fourth aspect, the present invention provides methods for algaegrowth, particularly microalgae, comprising culturing algae in thephotobioreactor of the first or second aspects of the invention, in thepresence of nutrient sources and light, preferably sunlight. Microalgae(for short, algae) are micro-plants and require mostly simple mineralnutrients for growth and reproduction. By utilizing photon energy, suchas sunlight and artificial illumination, algae convert, throughphotosynthesis, water and carbon dioxide into high-value organiccompounds (e.g., pigments, proteins, fatty acids, and secondarymetabolites). Accompanied by photosynthesis, nitrogen and phosphorous inwastewater are also taken up and assimilated in algal cells. Algaeexhibit a growth potential an order of magnitude greater than higherplants because of their extraordinarily efficient light and nutrientutilization.

Nutrient sources for such algal growth include, but are not limited to,standard algal culture media, animal wastewater, wastewater fromconcentrated animal feeding operations, nutrient contaminatedgroundwater and/or agriculture runoff water can be used as growth mediumafter balancing with certain chemicals, including but not limited tophosphate and trace elements, underground saline water after spiked withcertain chemicals, including but not limited to nitrate and phosphate,industrial wastewater, domestic wastewater, and contaminatedgroundwater, as well as waste gases emitted from power generators thatburn algal biomass residues after desired products are extracted andrecovered, flue gas emissions from fossil fuel fired power plants, dairywastewater containing high concentrations of ammonia and phosphate,groundwater of high nitrate levels.

As discussed above, the use of different nutrient concentrations affectthe growth and biochemical composition of algal cells. For example,nutrient-rich medium may stimulate and sustain a high growth rate andbiomass productivity, whereas nutrient depleted medium may stimulatebiosynthesis and cellular accumulation of neutral lipids, long chainfatty acids, and/or secondary carotenoids. A nutrient gradient createdin a photobioreactor of the first or second aspects of the inventionthus allows a continuous shifting of algae from a high biomassproduction mode to a high accumulation of specific desired product mode.

In a preferred embodiment for use in the methods of this fourth aspectof the invention, the photobioreactors of the first or second aspect areused and are placed on a berm, such as a mound or bank of earth, that israised and sized to provide the appropriate inclination of thephotobioreactor relative to the sun for maximum harvesting of solarradiation, as discussed above.

This fourth aspect further comprises methods for harvesting algal cellsfrom culture suspension maintained in the photobioreactors of theinvention, using various approaches (e.g., centrifugation, dissolved airflotation, or membrane filtration). Industrial preparative centrifugesof several kinds can be used to harvest algal cells from thephotobioreactors. These methods can be used to produce high-value algalproducts, such as long chain unsaturated fatty acids, carotenoids,phycobiliproteins, chlorophyll, and polysaccharides. A dissolved airflotation method for harvesting algal cells from a reactor can also beused. (Hoffland Environmental, Inc. 10391 Silver Springs Road, ConroeTex. 77303-1602; SAMCO Technologies, Inc. 160 Wales Avenue, PO Box 906,Tonawanda, N.Y. 14150) Such method is particularly useful in a processwhere treatment of wastestream (such as waste nutrients and CO₂) iscoupled with algal biomass production. A membrane filtration can also beused to harvest algal cells. (e.g., US Membranes Corp.)

The methods of this fourth aspect of the invention can be used for avariety of purposes, including but not limited to production ofalgae-derived products (pharmaceuticals, nutraceuticals, agrochemicals,human food, and animal feed); production of recombinant proteins;environmental remediation, including but not limited to removal andrecycling of waste nutrients from wastewater and carbon dioxide-richflue gases emitted from fossil fuel-fired power generators; andproduction of algal biomass for use as feedstock and for production ofbiofuels (such as biodiesel, ethanol, or methane), animal feedadditives, and organic fertilizers.

Those of skill in the art are familiar with standard algal culturemethods in photobioreactors. See, for example, Gitelson et al., Appliedand Environmental Microbiology 62: 1570-1573 (1996); Hu and Richmond(1994) Journal of Applied Phycology 6: 391-396; Hu et al., (1998)Journal of Fermentation and Biotechnology 85: 230-236; Hu et al., (1996)Biotechnology and Bioengineering 51: 51-60; Hu et al., (1996) Journal ofPhycology 32: 1066-1073; Hu et al., (1998) Applied Microbiology andBiotechnology 49: 655-662; Hu et al., (1998) European Journal ofPhycology 33: 165-171; and Hu, Industrial production of microalgalcell-mass and secondary products—major industrial species: Arthrospira(Spirulina) platensis. Pp. 264-272. In: Richmond A. (ed.) Handbook ofmicroalgal culture: biotechnology and applied Phycology, BlackwellScience Ltd., Oxford, UK.

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1. A photobioreactor comprising: (a) a container adapted for holding fluid, comprising (i) opposing first and second sidewalls, wherein at least one of the first and second sidewalls is transparent; (ii) opposing first and second endwalls; (iii) a container bottom; and (iv) a container cover, wherein the first and second sidewalls comprise a plurality of separate sections, and wherein the separate sections are in fluid communication; (b) support struts for connecting the plurality of separate sections of the first and second sidewalls; (c) at least one inlet port in fluid communication with the container; (d) at least one outlet port in fluid communication with the container; (e) an aeration system in fluid communication with the container; (f) a temperature control system connected to the container so as to control temperature of fluid within the container and (g) at least one baffle connected to the first and second sidewalls, so as to form a barrier and to partially separate the container into multiple compartments; wherein said at least one baffle comprises at least one valve configured to allow selective fluid communication between said plurality of separate sections.
 2. The photobioreactor of claim 1, wherein the first and second sidewalls comprise at least 5 separate sections.
 3. The photobioreactor of claim 1, wherein the first and second sidewalls comprise at least 50 separate sections.
 4. The photobioreactor of claim 1, wherein the number of support struts is equal to (n +1), wherein n is the number of first and second sidewalls.
 5. The photobioreactor of claim 1, wherein a distance in the container between the two sidewalls is between 100 millimeters and 30 centimeters.
 6. The photobioreactor of claim 1, further comprising a drainage outlet in fluid communication with the container.
 7. The photobioreactor of claim 1, wherein said at least one valve comprises a solenoid-controlled valve.
 8. A photobioreactor module, comprising two or more photobioreactors of claim 1, wherein the containers of each photobioreactor are in fluid communication. 9-32. (canceled) 